1. Field of the Invention
The present invention relates to a filament for an X-ray tube, and more specifically to a coiled filament with an improvement in temperature distribution uniformity along the longitudinal direction of the filament. The present invention also relates an X-ray tube having such a filament.
2. Description of the Related Art
A coiled filament for an X-ray tube preferably gives itself a uniform temperature distribution as far as possible over the whole length of the filament. The ordinary coiled filament for an X-ray tube has a constant wire diameter and a constant coil pitch, and therefore its temperature becomes highest at the longitudinal center and drops in the vicinity of the both ends. If the temperature distribution of the filament is uniform, the intensity distribution of an electron beam emitted from the filament becomes uniform, so that the brightness distribution of an X-ray focus becomes uniform, the X-ray focus being made by the electron bombardment on the target (i.e., the anode) of an X-ray tube. In addition, if the temperature distribution of the filament is uniform, the amount of wire diameter wear of the coil becomes uniform as compared with a filament which is not uniform in temperature distribution, so that the lifetime is prolonged. Furthermore, if the temperature distribution of the coil is uniform, the maximum temperature of the filament can be lowered for obtaining the same X-ray tube current as compared with the filament which is not uniform in temperature distribution, so that the lifetime is prolonged as well.
While the present invention is concerned with a longitudinal variation in coil shape of the filament for an X-ray tube, the prior art most relevant thereto is disclosed in Japanese Utility Model Publication No. 6-9047 U (1994), which will be referred to as the first publication.
The first publication discloses that a filament for an X-ray tube has a particular coil pitch which is dense in the vicinity of the center and sparse in the vicinity of the both ends, so that the temperature in the vicinity of the center of the filament rises to make the electron density distribution Gaussian. It is considered accordingly that the prior art filament does not make the temperature distribution uniform but rather makes the temperature in the vicinity of the center higher than the ordinary coil having a constant coil pitch. The coiled filament of the first publication is 80 turns per inch in coil pitch in the vicinity of the center and 50 turns per inch in the vicinity of the both ends for example.
On the other hand, in the technical field other than the X-ray tube, a longitudinal variation in wire diameter of a coiled filament is known and disclosed for example in Japanese Patent Publication No. 58-26144 B (1983), which will be referred to as the second publication.
The second publication relates to a lamp for the fixing unit of a copying machine and discloses a longitudinal variation in wire diameter of a coiled filament. More specifically, the wire diameter of the filament is reduced at the longitudinal ends than at the longitudinal center, so that the heating value is increased at the longitudinal ends to raise the irradiance at the ends than at the center. A method of reducing the wire diameter at the ends is disclosed in the second publication and is the electropolishing method. The second publication also mentions a continuous variation of the wire diameter, which is realized by moving up and down the liquid level of the electropolishing solution or by moving up and down the filament so as to vary the dipping time of the filament in the electropolishing solution depending on the longitudinal position of the filament.
It would be thought of from the knowledge of the second publication that in the filament also for the X-ray tube the wire diameter is preferably reduced at the longitudinal ends to raise the heating value at those regions, so that the longitudinal temperature distribution becomes uniform. Even if the wire diameter is reduced, however, there would remain a peculiar problem specific to the X-ray tube filament, which will be explained below.
In the second publication, the electropolishing method is used for reducing the wire diameter at the longitudinal ends, the method being as follows: a glass vessel is filled with ten-percent aqueous sodium hydroxide solution as the electropolishing solution; a tungsten plate is immersed in the solution to be the electrode; only the end region of the filament is immersed in the solution; and a voltage is supplied between the filament and the electrode so that the end region is electropolished. With this method, the wire diameter is certainly reduced at the longitudinal ends, but the coil outside diameter is also reduced at the longitudinal ends together with the reduction of the wire diameter. Accordingly, the filament made with such a method has a coil outside diameter which is smaller at the longitudinal ends than at the longitudinal center.
In the filament for the X-ray tube, if the outside diameter of the coiled filament varies along the longitudinal direction, the following problem is raised. The distance between the target and the filament in the X-ray tube affects the course of the electron beam traveling from the filament to the target and affects also the brightness of the X-ray focus on the target. Stating in detail, if the coil outside diameter varies along the longitudinal direction of the filament, the distance described above varies delicately, so that it adversely affects the brightness distribution of the X-ray focus. Therefore, it is important for the X-ray tube filament to keep the coil outside diameter constant along the longitudinal direction. For the reasons mentioned above, the known countermeasure, which is reduction of the wire diameter at the longitudinal ends so as to make the longitudinal temperature distribution uniform, disclosed in the second publication would not be applicable to the X-ray tube filament in a general way.